Configurations of interconnected arrays of conductive elements, as used, for example, in programmable logic gate arrays, requires the formation of conductive links, or paths, between selected conductive elements in a manner which produces relatively low resistance links between them. Techniques for producing such low resistance conductive links have been developed using either electrical or laser linking and cutting processes.
The latter laser processes have been preferred for certain applications because they provide permanent links and require no programming wiring or high voltage switching on the chip. Laser programmable techniques have the potential for providing higher performance and greater link density than electrical techniques if the linking device itself is sufficiently small. Ultimately the minimum size laser link would be a simple crossing of two wires. However, up to now insofar as is known, a successful process does not exist for providing such links. A primary concern when using any linking technology is the ability to use standard processing for the metal lines on the insulation. More specifically, this means the ability to integrate laser restructurable elements using standard silicon based MOS processing without the need to incorporate additional steps. Lateral links, which produce conductive links using silicon diffusion, have been used for some time to achieve compatibility with CMOS processing, as disclosed in U.S. Pat. No. 4,455,495, to Masuhara et al. and in U.S. Pat. No. 4,937,475, issued to F. M. Rhodes et al. on Jun. 26, 1990. These techniques require large areas to focus the laser to the substrate and have high resistance.
Other recent exemplary techniques have been proposed using laser linking processes for interconnecting metal layers at different levels. One such technique is disclosed in U.S. Pat. No. 5,166,556 issued on Nov. 24, 1992 to F. Shu et al. in which a laser beam is applied to an upper titanium metal layer at the location at which a link is desired to be made with a lower titanium layer. Laser power is supplied at a sufficient level to cause a chemical reduction reaction between the titanium layers and the intermediate silicon dioxide insulating layer so as to produce a conductive compound between the titanium layers which acts as an electrically conductive circuit therebetween. Such technique requires additional non-standard process steps and produces high resistance links and, hence, low performance.
U.S. Pat. No. 4,810,663 issued to J. I. Raffel et al. on Mar. 7, 1989 discusses a technique in which a diffusion barrier layer is placed between each metal layer and the insulation layer and the link region is exposed to a low power laser for a relatively long time (i.e., a relatively long pulse width) to cause the metals to alloy with the diffusion and insulating layers to form the desired conductive link. Such technique requires a relatively long laser power pulse output using a relatively complicated diffusion barrier/insulation structure so as to produce an opening in the upper layer to permit the energy to be applied to the barrier and insulating layers to produce the desired alloying operation.
A further technique has been proposed to provide lateral links between metals substantially at the same surface or plane as discussed in U.S. Pat. No. 4,636,404 issued to J. I. Raffel et al. on Jan. 13, 1987. Again relatively long pulses are applied to the general region between the metals so as to cause the metals to form an aluminum doped silicon link.
In a recent article "Laserpersonalization of Interconnection Arrays for Hybrid" ASIC's of M. Burnus et al., IEEE International Conference on Wafer Scale Integration, 1993, a laser beam is used to provide sufficient power to blast a hole through an upper metal layer so as to form an opening at the link region. Multiple laser pulses of high energy density are used to create the opening and to remove the insulating layer between the metal elements. The multiple pulses also produce molten aluminum which spreads along the walls of a crater that is formed when the insulating layer is removed beneath the opening. Such aluminum flow along the crater walls produces a tube-like aluminum contact body between the upper and lower aluminum layers.
The article "Laser Programmable Vias for Reconfiguration of Integrated Circuits" by Rouillon-Martin et al. in Optical Microlithography and Metrology for Microcircuit Fabrication, 1989, discloses a technique which performs a similar operation to that discussed in the above Burnus et al. article in which the opening is made much smaller in diameter by using multiple pulses of a relatively highly focused laser beam.
It is desirable to devise a laser linking process which produces a link structure between any two metal layers which can be fabricated in a manner which is compatible with standard MOS processes and which provides high performance (low resistance) and high density (small area) links. Such process should use relatively low laser power and provide self-contained links with low peripheral damage at the link sites.